Outline Week 13 Overview: Initiation of Division Cancer: A Failure of Control over Cell Division The Normal Control of Cell Division Somatic Mutation and the Genetics of Cancer Copyright © The McGraw-Hill Companies, Inc Permission required to reproduce or display Hartwell et al., 4th edition, Chapter 17 Overview of Cell division and cell cycle How cancer can arise? Proliferation of cells Some cells divide out of control, spread (metastasize) excessive and inaccurate proliferation cells grow in invasive way In human adults: 300 different types of cells divide when needed Rarely occurs in children, common in older adults • inner skin, blood, intestinal lining: daily • liver cells rarely divide • nerves, surface skin: never divide In normal conditions, cell division is under the control, in balance and tight organization Figure: Lung cancer cells (530x) These cells are from a tumor located in the alveolus (air sac) of a lung The relative percentages of new cancers in the United States that occur at different sites in the bodies of men and women Overview of Cancer Cancer is a disease of genes: mutations in genes that regulate cell cycle (growth and division) Environmental factors (chemicals ) raises the rate mutation Cancer differs in two ways: most mutations in some somatic cells • accumulate over time (sporadic) • inheritant mutations predisposition to cancer mutations in germline cells: mutations in all cells of all somatic tissues • Examples: cystic fibrosis, Huntington disease Fig 17.1 Copyright © The McGraw-Hill Companies, Inc Permission required to reproduce or display Hartwell et al., 4th edition, Chapter 17 Two unifying themes about cancer genetics The initiation of cell division Cancer is a disease of genes Two basic types of signals that tell cells whether to divide, metabolize or die • Multiple cancer phenotypes arise from mutations in genes that regulate cell growth and division Extracellular signals – act over long or short distances • Environmental chemicals increase mutation rates and increase chances of cancer • Steroids, peptides, or proteins • Collectively known as hormones Cancer has a different inheritance pattern than other genetic disorders Cell-bound signals • histocompatibility proteins • require direct contact between cells • Inherited mutations can predispose to cancer, • The mutations causing cancer occur in somatic cells • Mutations accumulate in clonal descendants of a single cell An example of an extracellular signal that acts over large distances An example of an extracellular signal that is mediated by cell-to-cell contact Thyroid-stimulating hormone (TSH) produced in pituitary gland Moves through blood to thyroid gland, which expresses thyroxine Fig 17.2b Fig 17.2a 10 Hormones transmit signals into cells through receptors that span the cellular membrane Each signaling system has four components Growth factors • Extracellular hormones or cell-bound signals that stimulate or inhibit cell proliferation Receptors • Have three parts: a signal-binding site outside the cell, a transmembrane segment, and an intracellular domain Signal transducers • Located in cytoplasm, relaying the signal inside the cell Transcription factors Fig 17.3a • Activate expression of specific genes to either promote or inhibit cell proliferation 11 12 Signaling systems can stimulate or inhibit growth RAS is an intracellular signaling molecule Signal transduction activation or inhibition of intracellular targets after binding of growth factor to its receptor Fig 17.3d Fig 17.3b&c 13 14 Cancer phenotypes result from the accumulation of mutations Outline Mutations are in genes controlling proliferation as well as other processes Overview: Initiation of Division Cancer: A Failure of Control over Cell Division The Normal Control of Cell Division • Result in a clone of cells that overgrows normal cells Cancer phenotypes include: • Uncontrolled cell growth • Genomic and karyotypic instability • Potential for immortality • Ability to invade and disrupt local and distant tissues 15 16 Phenotypic changes that produce uncontrolled cell growth Phenotypic changes that produce uncontrolled cell growth (cont) Most normal cells Most normal cells Autocrine stimulation: Cancer cells can make their own stimulatory signals a.1 Most normal cells Loss of contact inhibition: Loss of cell death: Cancer cells are more resistant to programmed cell death (apoptosis) Many cancer cells Many cancer cells Loss of gap junctions: a.2 Many cancer cells a.3 Most normal cells Many cancer cells a.4 Cancer cells lose channels for communicating with adjacent cells Growth of cancer cells doesn't stop when the cells contact each other Fig 17.4 Fig 17.4 17 18 Phenotypic changes that produce genomic and karyotypic instability Phenotypic changes that produce genomic and karyotypic instability b.1 Defects in DNA replication machinery: Increased rate of chromosomal aberrations: Cancer cells have lost the ability to replicate their DNA accurately Cancer cells often have chromosome rearrangements (translocations, deletions, aneuploidy, etc) Increased mutation rates can occur because of defects in DNA replication machinery Some rearrangements appear regularly in specific tumor types Fig 17.4b.2 Fig 17.4 19 20 Phenotypic changes that enable a tumor to disrupt local tissue and invade distant tissues Phenotypic changes that produce a potential for immortality Loss of limitations on the number of cell divisions: Tumor cells can divide indefinitely in culture (below) and express telomerase (not shown) Ability to metastasize: Tumor cells can invade the surrounding tissue and travel through the bloodstream d.1 d.2 Angiogenesis: Tumor cells can secrete substances that promote growth of blood vessels Fig 17.4 Fig 17.4 21 Multiple mutations leading to convert a normal cell into a cancerous cell DNA sequencing revealed thousands of mutations in each tumor How many actually contribute to the cancer phenotype is unclear Identify and isolate a mutation of interest by 22 Evidence from mouse models that cancer is caused by several mutations mutations Transgenic mice with dominant mutations in the myc gene and in the ras gene (a) Mice with recessive mutations in the p53 gene (b) linkage analysis of markers, traditional genetic mapping to a chromosome, and positional cloning Use gene transfer experiments in mice to test whether a mutation in a single gene associated with cancer is sufficient to induce a tumor the mutation acts in a dominant or recessive fashion Fig 17.5 Fig 17.5a 24 Evidence that cancer cells are clonal descendants of a single somatic cell Analysis of polymorphic enzymes The role of environmental mutagens in cancer encoded by the X chromosome in female Concordance for the same type of cancer in first degree relatives (i.e Sample from normal tissues has The incidence of some cancers varies between countries (see Table 17.1) siblings) is low for most forms of cancer mixture of both alleles • When a population migrates to a new location, the cancer profile becomes like that of the indigenous population Clones of normal cells has only one allele Numerous environmental agents are mutagens and increase the likelihood of Sample from tumor has only one cancer allele • Some viruses, cigarette smoke Fig 17.6 25 The incidence of some common cancers varies between countries 26 Cancer development over time Lung cancer death rates and incidence of cancer with age Table 17.1 Fig 17.7 27 28 Cancer is thought to arise by successive mutations in a clone of proliferating cells Some families have a genetic predisposition to certain types of cancer Example: retinoblastoma caused by mutations in RB gene Individuals who inherit one copy of the RB− allele are prone to cancer of the retina During proliferation of retinal cells, the RB+ allele is lost or mutated Tumors develop as a clone of RB−/RB− cells Fig 17.8 Fig 17.9 29 30 Cancer producing mutations are of two general types Cancer Some important definitions Cancer genes: the mutant alleles of normal genes that lead to cancer Mutant alleles that act dominantly are known as oncogenes; The wild-type genesthat become oncogenes upon mutation are known as proto-oncogenes Mutant alleles that act recessively are known as mutant tumor-suppressor genes Fig 17.10 31 Cancer-causing retroviruses carry a mutant or overexpressed copy of a Cancercellular gene Oncogenes act dominantly and cause increased proliferation After infection, retroviral genome integrates into host genome Oncogenes are produced when mutations cause improper activation a gene If the retrovirus integrates near a proto-oncogene, the proto-oncogene can be packaged with the viral genome and become mutated Two approaches to identifying oncogenes: • Tumor-causing viruses (Fig 17.11a) Many tumor viruses in animals are retroviruses Some DNA viruses carry oncogenes [e.g Human papillomavirus (HPV)] • Tumor DNA (Fig 17.11b) Transform normal mouse cells in culture with human tumor DNA Fig 17.11a 33 34 DNA from human tumor cells is able to transform normal mouse cells cells into tumor cells Retroviruses and their associated oncogenes a virus carrying one or more oncogenes infects a cell, the oncogenes cause abnormal proliferation cells lead to the accumulation of more mutations cancer Human gene that is oncogenic can be identified and cloned from transformed mouse cells Fig 17.11b Table 17.2 35 36 The RAS oncogene is the mutant form of the RAS protoproto-oncogene Oncogenes are members of signal transduction systems Normal RAS is inactive until it becomes activated by binding of growth factors to their receptors Oncogenic forms of RAS are constitutively activated (GTP-activated form) Fig 17.11c Table 17.3 37 Mutations inactivate tumor suppressor genes cause cancer 38 The retinoblastoma tumor-suppressor gene Function of normal allele of tumor suppressor genes is to control cell proliferation Mutant tumor suppressor alleles act recessively and cause increased cell proliferation One wild-type copy produces enough protein to regulate Tumor suppressor genes identified through genetic analysis of families with inherited predisposition to cancer • Inheritance of a mutant tumor suppressor allele • One normal allele sufficient for normal cell proliferation in heterozygotes • Wild-type allele in somatic cells of heterozygote can be lost or mutated abnormal cell proliferation Fig 17.12 39 The retinoblastoma tumor-suppressor gene 40 Mutant alleles of these tumortumor-suppressor genes decrease the accuracy of cell reproduction Mutant allele of RB gene is recessive How can the retinoblastoma trait be inherited in a dominant fashion if a deletion of the RB gene is recessive to the wild-type RB allele? Because: in many retina heterozygous cells, only one cell can have mutation at single remaining RB allele a clone of cancerous cells The recessive RB mutation that leads to retinoblastoma through the genomic analysis of families inheriting a predisposition to the cancer Table 17.4 42 The normal control of cell division Outline Overview: Initiation of Division Cancer: A Failure of Control over Cell Division The Normal Control of Cell Division Four phases of the cell cycle: G1, S, G 2, and M Copyright © The McGraw-Hill Companies, Inc Permission required to reproduce or display Hartwell et al., 4th edition, Chapter 17 Fig 17.13 43 44 Experiments with yeast helped identify genes that control cell division division Two kinds of used: Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast) Usefulness of yeast for studies of the cell cycle • Both grow as haploids or diploids Can identify recessive mutations in haploids Can complementation analysis in diploids • S cerevisiae – size of buds serves as a marker of progress through the cell cycle Daughter cells arise as small buds on mother cell at end of G1 and grow during mitosis Stage of cell cycle can be determined by relative appearance of buds (see Fig 17.14) 46 A temperaturetemperature-sensitive cellcell-cycle mutant in S cerevesiae The isolation of temperaturetemperature-sensitive mutants of yeast Cells grown at permissive temperature display buds of all sizes (asynchronous division) Mutants grow normally at permissive temperature (22°) Growth of the same cells at restrictive temperature – all have large buds At restrictive temperature (36°), mutants lose gene function After replica plating, colonies that grow at 22° but not at 36° have temperaturesensitive mutation Fig 17.14a Fig 17.14b Fig 17.15 47 48 CDKs interact with cyclins and control the cell cycle by phosphorylating other proteins Some important cellcell-cycle and DNA repair genes Cyclin-dependent kinases (CDKs) – family of kinases that regulate the transition from G1 to S and from G2 to M • Cyclin specifies the protein targets for CDK Phosphorylation by CDKs can activate or inactive a protein Fig 17.16a Table 17.5 49 50 CDKs control the dissolution of the nuclear membrane at mitosis CDKs function only after associating with a cyclin Cyclin specifies which set of proteins a CDK phospholylates Cyclins are unstable and their levels are regulated strictly Example: CDK phosphorylate to active the nuclear lamins Nuclear lamins – provide structural support to the nucleus • Form an insoluble matrix during most of the cell cycle At mitosis, lamins are phosphorylated by CDKs and become soluble Fig 17.16 52 CDKs mediate the transition from the G1 to the S phase of the cell cycle Mutant yeast permit the cloning of a human CDK gene Human CDKs and cyclins can function in yeast and replace the corresponding yeast proteins Fig 17.17 53 Fig 17.18 54 CDK activity in yeast is controlled by phosphorylation and dephosphorylation Cell-cycle checkpoints ensure genomic stability Checkpoints monitor the genome and cell-cycle machinery before allowing progression to the next stage of cell cycle G1-to-S checkpoint • DNA synthesis can be delayed to allow time for repair of DNA that was damaged during G1 The G2-to-M checkpoint • Mitosis can be delayed to allow time for repair of DNA that was damaged during G2 Spindle checkpoint • Monitors formation of mitotic spindle and engagement of all pairs of sister chromatids Fig 17.19 55 56 Disruption of the G1-to to S checkpoint in p53 deficient cells can lead to amplified DNA p53 The G1-to to S checkpoint is activated by DNA damage Tumor cells often have homogenously staining regions (HSRs) or small, extrachromosomal pieces of DNA (double minutes) Fig 17.20a Fig 17.20b 57 Disruption of the G1-to to S checkpoint in p53 deficient cells can lead to many types of chromosome rearrangements p53 rearrangements 58 Checkpoints acting at the G2-to to M cellcell-cycle transition or during M phase Fig 17.21 Fig 17.20c 59 60 10 Three classes of error lead to aneuploidy in tumor cells The necessity of checkpoints Checkpoints are not essential for cell division Cells with defective checkpoints are viable and divide at normal rates • But, they are much more vulnerable to DNA damage than normal cells Checkpoints help prevent transmission of three kinds of genomic instability (Fig 17.22) • Chromosome aberrations • Changes in ploidy • Aneuploidy Fig 17.22a 61 62 Chromosome painting can be used to detect chromosome rearrangements rearrangements Chromosomes from tumor cells Chromosomes from normal cells Fig 17.22 63 11 ... unifying themes about cancer genetics The initiation of cell division Cancer is a disease of genes Two basic types of signals that tell cells whether to divide, metabolize or die • Multiple cancer. .. descendants of a single somatic cell Analysis of polymorphic enzymes The role of environmental mutagens in cancer encoded by the X chromosome in female Concordance for the same type of cancer in... cells • Inherited mutations can predispose to cancer, • The mutations causing cancer occur in somatic cells • Mutations accumulate in clonal descendants of a single cell An example of an extracellular